The desire to treat surfaces has been as old as mankind. These desires may be inspired by changing a variety of surface properties, ranging from the aesthetic over the physical to the chemical properties.
In many applications the mechanical, chemical or physical properties of surfaces of materials play an important role. If, for any reason, the requirements can not be met by the bulk of the material, the application of coatings and surface modification is a convenient method in order to improve the properties. In this way many substrates can be refined and used in new applications. A very simple case is the refinement by coating with decorative and coloured layers.
Also from an industrial point of view, there is an increasing interest to control the surface functionality and the surface properties of all kinds of substrates. More and more demands arise for controlling for example adhesion and release properties of different substrates, more specifically of polymer substrates. These properties are often linked to the hydrophobic or hydrophilic character of the surface. Others may be interested in making surfaces oil or water repellent, for instance of fabrics but also of metal, glass, ceramics, paper, or polymerics, for purposes such as improving preservation properties, or to prevent or inhibit soiling of the surfaces.
Conventional methods for the modification of surface properties involve the use of wet chemical deposition of coatings on a given substrate, often commonly called “painting”. However, most of the time solvents have to be used as the basis for the paint, for carrying the chemicals which must form the coating onto the surface. This has led to increased costs, industrial health and environmental issues because of the volatility of the solvents, which are contributing to problems such as sick-building-syndrome because of the emission of volatile organic compounds (VOC) into the atmosphere. For those reasons, paints based on organic solvents have found more and more substitution by water-based paints. This has been able to reduce the problem but has not necessarily been able to avoid all VOC emissions. There has therefore remained a need for improved means for modifying the properties of various surfaces.
Standard industrial coating technologies now may comprise the application of a lacquer followed by thermal or UV-induced curing treatment.
Techniques involving vacuum have also been developed, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and low pressure plasma techniques.
A widely used method for surface modification of a polymer is corona treatment at atmospheric pressure. The drawback however is that this technique produces inhomogeneous surface changes and that the changes are not stable in time. Corona treatment may optionally be combined with a wet chemical deposition.
WO2005/089957 describes the use of atmospheric pressure plasma for the pretreatment of a given substrate, after which a solvent containing reactive solution is applied to form a stable coating after solvent removal.
Another commonly used method for the modification of surface properties of a substrate and/or to produce coatings on a substrate is to submit the substrate to a low pressure plasma treatment. In particular, it is known to use a polymer forming precursor, which may also be called a monomer, as the coating forming material, and to introduce said precursor into a plasma discharge, whereby polymerization takes place to form a polymer coating on the substrate.
WO 2007/053916 describes an improvement of this technique wherein a doping or dedoping agent is added as second component into the plasma containing a conjugated polymer forming precursor such as thiophene or a derivative thereof. The second component may be an inorganic or mixed organic/inorganic precursor so that a hybrid organic/inorganic coating is formed.
WO00/032248 describes the enhancement of the surface properties of polymers for medical applications using low pressure plasma in the presence of various gaseous precursors or hydrocarbons.
US2004/258931 describes the plasma cross-linking at low pressure of a double bond containing monomer on a hydrophilic polymeric layer deposited on the substrate to enhance the hydrophilicity of medical devices.
WO95/04609 describes the deposition of a hydrophilic coating on a lens by low-pressure plasma technology using a carrier gas comprising hydrogen peroxide and at least one functional organic compound.
However, low-pressure plasma has the disadvantage of requiring very complex and thus highly cost-ineffective reactors and therefore large investments for industrializing the process. In addition, low-pressure plasma processes are generally batch processes which are very difficult to integrate into existing continuous production facilities.
Recent developments in the field of atmospheric plasma technology are creating new perspectives beyond current state of the art in corona pretreatment of materials. By controlling the gas atmosphere and electrical conditions, one may increase the efficiency of the plasma surface treatment significantly. Furthermore, by adding reactive chemical precursors to the plasma discharge, the surface chemistry may be controlled and thin functional coatings may be deposited. In document “Production and deposition of adsorbent films by plasma polymerization on low cost micromachined non-planar microchannels for preconcentration of organic compound in air”, by Lima et al, Sensors and Actuators, vol. 108, nr 1-2 (2005), a plasma deposition of ethyl acetate is described, resulting in a hydrophilic film.
Plasma grafting may also be employed to enhance the surface hydrophilicity of polymers. It is usually conducted by first exposing a polymer to a plasma such as argon, helium, or nitrogen, for a short time, typically a few seconds. This part of the process introduces many radicals to the surface of the polymer. Afterwards, the polymer substrate is brought into contact with the vapour of a monomer or with air. However, in this case, the improved properties are not necessarily stable in time, and the substrate tends to come back rather quickly to its original state.
Advantages of the ambient pressure methods are that they may be used with standard and inexpensive coating and curing equipment. Vacuum methods are often associated with higher complexity equipment, and thus with additional costs concerning equipment and processing. Therefore, in metallisation processes and coating of small and high value substrates, e.g. ophthalmic lenses, the vacuum methods have been found to be successful.
In many cases and for special applications also other functional properties may have to be improved, e.g. hardness, chemical resistance, electrical resistivity, barrier properties or optical appearance.
EP 1557489 describes how the surface of a polymeric substrate or glass is made water and/or oil repellent by exposing the surface to a plasma comprising a perfluoroalkene or polyfluoroacrylate vapour, and causing plasma polymerization onto the surface.
WO 2005/095007 A1 discloses that hybrid organic/inorganic hybrid pre-polymers, formed via sol-gel processing, may also be added to the plasma discharge. This simple plasma-curing technique leads to additional cross-linking in the coating as compared to previously known techniques.
WO 2009/037331 A1 describes how a substrate surface may be given a stable hydrophilic coating by treatment in an atmospheric plasma into which ethyl acetate is injected in the form of an aerosol.
WO 2007/021180 discloses the growing of highly branched polymers called “polymer brushes” with a high density onto a surface which may have been activated using chemisorption reactions, using the “grafting to” technique by contacting the activated surface with polymer chains in the form of a melt.
In EP 1095711 A2 this technique is refined by first plasma coating the surface with 1,2-Diaminocyclohexane such that the surface is provided with polymer brushes having amino functions. These are then used to bind a polymerization initiator, which then allows graft polymerization of ethylenically unsaturated hydrophilic monomers or macromonomers onto the surface, such that the treated surface has an improved wettability, water retention ability and biocompatibility.
WO 03/086031 discloses an atmospheric plasma process comprising the spraying of liquid precursors in a plasma causing polymerization.
WO 2006/053403 A2 discloses a method for the immobilization of chitosan on a surface, substantially resistant to leaching and having strong microbial activity. The biaxially oriented polypropylene polymeric surface was activated before addition of chitosan to the surface by a surface plasma-activation at atmospheric pressure.
WO 2005/106477 discloses a method for the inclusion and immobilisation of biological molecules into a thin plasma polymerized and deposited coating layer using a single step process, with the purpose of obtaining bio-engineered materials having bio-recognition sites which may still interact with other species of interest, including biological species, and which may be useful in a wide range of applications. WO 2005/106477 is concerned with keeping the biological functionality of the bound biomolecules intact.
Boulares-Pender, A. et al, in “Surface-Functionalization of Plasma-Treated Polystyrene by Hyperbranched Polymers and Use in Biological Applications”, Journal of Applied Polymer Science, Vol. 112, 2701-2709 (2009), Wiley Periodicals, Inc, disclose a method to increase the functionality density of a surface by in-situ growing or building of hyperbranched structures from a nitrogen plasma-exposed polystyrene (PS) surface, which may have reacted with atmospheric oxygen after plasma treatment. The plasma-exposed PS surfaces showed an increase of their wettability due to the formation of polar functional groups, as evidenced by contact angle measurements. The subsequent building of hyperbranched structures used 2-aminoethyl methacrylate hydrochloride (AEMA), glutaraldehyde (GA) and tetra-ethylene pentamine (TEPA) and built up to three generations of GA-TEPA segments on the surface. The advantage is that any biomolecules later on being attached to the surface are more likely to find a higher number of anchor points on the hyperbranched segments grown onto the PS surface. This method involves several process steps, and is therefore rather complex.
The density of the functional groups on the surfaces obtained with these known processes may remain however limited, and also the number of points with which the molecule fractions containing the functional groups are attached to the surface may remain limited, so that the obtained surface properties and the stability of these properties over time remain short of what may be desired.
There therefore remains a need for a simple technique which is able to deposit functional groups on a surface with a high density, such that the surface properties and the stability of these properties over time are improved as compared to what may be obtained with the so-far known techniques.
The present invention aims to obviate or at least mitigate the above described problems and/or to provide improvements generally.